Refs #2068: Do not reinstantiate staticfiles storage classes

[matthiask]

Description

This change might address an issue where reinstantiating the staticfiles class inside our staticfiles panel would lose some attributes of the storage instance.

Fixes #2068

Checklist:

• I have added the relevant tests for this change.
• I have added an item to the Pending section of docs/changes.rst.

[tim-schilling]

@matthiask would it be possible to wrap the url method on the storage class without having to create a new class?

[matthiask]

@tim-schilling Yes, I think we could directly monkey-patch the url method on the static files storage instance. Do you think that would be preferrable to the Mixin injection? I can see how a straightforward monkeypatch might be easier to understand.